Device for determining relative angular position between a spacecraft and a radiation emitting celestial body

ABSTRACT

Signals indicative of the relative angular position between a spin stabilized spacecraft, probe, or sounding rocket and a radiation emitting celestial body are derived with a detector including four electrodes for deriving indications of the centroid the radiation image on the detector. During each spin of the satellite each electrode derives a signal having a first non-zero level while the detector is not illuminated by the radiation and a sound non-zero level while it is illuminated by the radiation. The first level is indicative of dark current, while the second level is dependent upon dark current, the angular position of the centroid of the image on the detector surface relative to the electrode, and the intensity of the radiation impinging on the detector. A processing network, including a negative feedback loop, responds to the signal to derive, during each spin of the spacecraft, a signal indicative of the dark current. The dark current indicating signal is combined, in the feedback loop, with the electrode output to derive an output signal having a substantially zero value while the dark current is being generated. The image of the body is formed on the detector surface with a pinhole.

[ July 10, I973 DEVICE FOR DETERMINING RELATIVE PrimaryExaminer--Benjamin A. Borchelt ANGULAR POSITION BETWEEN A A ssis tantEgrargir zg$. C. Buczinski SPACECRAFT AND A RADIATION Attorney-R. F.Kempf, John R. Manning et al. EMITTING CELESTIAL BODY [75] Inventors:Winfield I-I. Farthing, Lanham, Md.; ABSTRACT Herbert Spnngfield Signalsindicative of the relative angular position be- [73] Assignee: TheUnited State of A i as tween a spin stabilized spacecraft, probe, orsounding represented by h Administrator f rocket and a radiationemitting celestial body are dethe N fi n l Aeronautics and Spam rivedwith a detector including four electrodes for dei nt Washington, DCriving indications of the centroid of the radiation image on thedetector. During each spin of the satellite [22] Filed 1972 eachelectrode derives a signal having a first non-zero [21] A l, M 229,128level while the detector is not illuminated by the radiation and a soundnon-zero level while it is illuminated by the radiation. The first levelis indicative of dark [52] Cl 356/141 250/214 current, while the secondlevel is dependent upon dark [51] Int Ci Golb 11/26 current, the angularposition of the centroid of the im- [58] Fie'ld "5 473' age on thedetector surface relative to the electrode, 250/214 356/141 and theintensity of the radiation impinging on the detector. A processingnetwork, including a negative [56] References Cited feedback loop,responds to the signal to derive, during each spin of the spacecraft, asignal indicative of the UNITED STATES PATENTS dark current. The darkcurrent indicating signal is 3,657,547 4/1972 Mansfield 250/203 RCombined, i the feedback loop, with the electrode output to derive anoutput signal having a substantially 2 538 028 1/1951 43/171 R zerovalue while the dark current is being generated. 34961481 2/1970 Toricket al. 325/473 The image of the body is formed the dfitector surfacewith a pinhole.

16 Claims, 3 Drawing Figures A4 CONTROL scawor 4Q TillGGElZ CONTROL TlMEMU LTl PLEXER CONTROLLER! l Patented July 10, 1973 3,744,913

2 Sheets-Sheet 1 DEVICE FOR DETERMINING RELATIVE ANGULAR POSITIONBETWEEN A SPACECRAFT AND A RADIATION EMITTING CELESTIAL BODY ORIGIN OFINVENTION The invention described herein was made by employees of theUnited States Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

FIELD OF INVENTION The present invention relates generally to devicesfor deriving signals indicative of the angular position between, aradiation emitting celestial body and a spacecraft, probe, or soundingrocket, and, more particularly, to such a device wherein a spacecraftincludes a detector for deriving signals indicative of the centroidposition of a radiation image on the detector surface. Hereafter, theword spacecraft is intended to include probes and sounding rockets.

BACKGROUND OF THE INVENTION Systems for determining the angular positionbetween a spacecraft and a radiation emitting celestial body, such asthe sun or moon, generally fall into three types; namely: analog sensorsincluding a pair of photoconductive devices, reticle time measuringdevices, and reticle digital devices.

Analog sensors employ a pair of photoconductive devices connected sothat a differential output is derived from them. The differential outputvaries as a function of the projection angle of the vector of theradiation image on a planar surface formed by the photoconductivesensors. To provide signals varying as a function of the angularposition of the radiant emitting celestial body relative to the detectorsurface, combinations of light baffles and sun shades are employed. Inorder to derive signals indicative of the angular position of theradiant source into two directions at right angles to each other,multiple sensors are required. Devices of this type require each of thephotoconductive sensors to be very closely matched in order to maintainaccuracy. Because of the requirement for multiple pairs ofphotoconductive devices to provide information indicative of the body intwo orthogonal directions, considerable thermal and mechanical stabilityof the photoconductive devices are necessary. It can be shown thathighly accurate determinations of the angular position of the radiatingsource are attained only if the field of view of the detector isrelatively narrow, less than ten degrees of arc. Further, detectors ofthe photoconductive type generally have an inherently slow response timeso that systems employing such detectors cannot usually be employed onspinning spacecraft. A further problem inherent with the use ofphotoconductive devices is that they cannot usually be employed withlenses, so that measurements of celestial bodies having relatively lowlight emission, such as the moon, are difficult.

The reticle time measuring system is applicable only to spinningspacecraft. A reticle containing a pattern of slits is located in frontof a photocell to provide radiation pulses from the celestial body onthe photocell in response to spin of the spacecraft. The time betweendetection of adjacent radiation pulses is measured to provide a measureof angular position. It is necessary to reference the detected timebetween adjacent pulses to the total spin period. While systems of thistype are relatively simple and inexpensive, it has been found that theaccuracy thereof is limited to measurements of only one degree of arc.

In the digital retical system, a slit retical is located in front of abinary or Gray-coded pattern of several photocell detectors. Theposition of the celestial body image is derived by the binary state ofthe several detectors in an image plane. For spinning vehicles, it isnecessary to employ an auxiliary detector to indicate when radiationfrom the celestial body is in the sensor field of view. To providemeasurements of the body in two orthogonal directions, it is necessaryto employ a pair of such detectors. While digital reticle systems areinherently precise, their resolution is limited as a'ru'n'- tion of thespectral nature of the radiant energy derived from the body. Forexample, radiation from the sun subtends an arc of 32 minutes which canbe detected with the digital type device. In order to obtain accuraciesto a greater extent, for example, to one arc minute, complex andexpensive interpolation schemes are required. A further problemconcerned with digital reticle systems is that measurements can bederived only once during each spin cycle when they are employed onspinning spacecraft. Thereby, the amount of information which can bederived with such devices is limited, so that indications of the motionof the vehicle are not easily derived.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the presentinvention, the abovenoted problems are avoided by employing asemiconductor detector having a fast response time and a plurality ofoutput electrodes which derive indications of the centroid of a radiantimage on the detector. The electrodes are responsive to currents derivedfrom a detecting surface preferably having substantial length in twodirections at right angles to each other. It has been found that withthe device of the present invention a carefully calibrated detector iscapable of yielding positional information in a pair of orthogonalcoordinate directions to accuracies within one arc minute.

In one preferred embodiment, the stated accuracy is attained over an arehaving a field of view of approximately 20. Such accuracy is attained inone embodiment through the use of a pinhole image former having an areano greater than approximately one-tenth the area of the detectorsurface. If the area of the pinhole is increased appreciably beyondone-tenth the area of the detector surface, system resolution isdecreased, precluding the ability to derive centroid information to oneminute of are. If the pinhole area is made excessively small, the amountof light reaching the detector surface is generally excessively low,whereby the signal to noise ratio of the detector output signal may dropto an intolerable level. While the pinhole configuration is preferredbecause of its simplicity and high resolution, in many instances otherimage forming devices, employing optical gain (such as collimatinglenses), may be employed.

A problem in processing the output signal of a detector is theappreciable dark current always derived at the output electrodes. Thedark current is stabilized to a certain extent by providing thermalcontrol for a substantially light tight housing in which the detector ismaintained. The detector dark current, however, is susceptible to changeas a function of time and other factors. Hence, it has generally beenfound that it is necessary to detect the dark current level and subtractit from the output signal of each electrode. To this end, the signalderived from each electrode is chopped periodically. If the spacecraftincludes means for maintaining it in a spin stabilized configuration,chopping is performed in response to the spacecraft spinning motion. Ifthe spacecraft is of the non-spin or very low spin type, chopping iseffected by rotating a reticle in front of the detector. While theradiation is being blocked due to the chopping action and the detectoris generating dark current, the level of the output signal of thedetector is determined. This level is combined in a feedback loop withthe electrode output signal to derive an output signal havingsubstantially zero level while no radiation from the source impinges onthe detector. While the detector is illuminated by the radiation, thefeedback loop output signal has a level commensurate with the intensityof the radiation impinging on the detector and the position of the imagecentroid relative to the electrode, with dark current offset removed. Inaddition to limiting the offset induced by the dark current, thistechnique provides greater dynamic range for the device because theratio of the information signal level to the signal level while thedetector is not illuminated becomes substantially infinite.

In accordance with one feature of the invention, the dark current is fedto the feedback loop only when the detector is not illuminated by theradiation. To this end, the feedback loop includes a storage capacitorfed through a normally closed switch by the output signal of a channelincluding the feedback loop. The switch is open circuited in response tothe detector providing an indication that it is being illuminated by theradiation.

A further feature of the invention involves sampling the output signalof each electrode channel several times during each spin of thespacecraft about the radiating body. Thereby, the motion of thespacecraft relative to the body is accurately ascertained in arelatively short time. Sampling is performed on a periodic basis,whereby each channel is sampled at the same time, Simultaneous samplingof all channels substantially avoids errors resulting from relativemotion between the spacecraft and the radiating body.

In accordance with still another feature of the invention, outputsignals from each of the electrode channels are fed to a single outputchannel via a time multiplexer. The single output channel is of thevariable gain type to enable the detector to be employed with radiationsources having different intensities. Thus, moon tracking can beperformed despite the significant changes in the intensity of radiationfrom the moon which result from its different phases. The use of asingle variable gain channel responsive to the several electrodechannels reduces the possibility of differing gain factors beingintroduced in the several channels.

It is, accordingly, an object of the present invention to provide a newand improved device for enabling the angular position between aradiating source and a spacecraft to be ascertained.

Another object of the invention is to provide a device for enabling therelative position between a radiating, celestial body to be determinedrelative to a spacecraft, wherein the spacecraft includes a detector forderiving signals indicative of the centroid position of an image of thebody on the detector surface.

A further object of the invention is to provide a relatively inexpensivedevice for enabling the position of a radiating celestial body to bedetermined on a spacecraft to within approximately one minute of arc.

A further object of the invention is to provide a new and improvedsystem for enabling the angular position of a radiation emittingcelestial body to be determined on a spacecraft, wherein the effects ofdark current on the output signal of a detector are substantiallyeliminated.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of one specific embodiment thereof,especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic diagramillustrating the position of the detector in accordance with the presentinvention on a spinning spacecraft relative to the sun;

FIG. 2 is a cross-sectional view of one preferred embodiment of adetector mounted on the spacecraft of FIG. 3; and

FIG. 3 is a circuit diagram of a preferred embodiment of the processingelectronics employed in conjunction with the detector of FIG. 2.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 ofthe drawing wherein there is illustrated a spin stabilized spacecraft 11including the usual means (not shown) for causing the spacecraft to spinabout its own vertical axis 12. The spacecraft is in orbit about a localcelestial body, such as earth, so that its angular position relative toa celestial body a great distance from it, such as the sun or moon, isvariable. To determine the angular position of spacecraft 11 relative tothe body, the spacecraft is provided with a radiation detector 13. Theoptical axis of the detector is elevated from the spin plane such thatthe celestial body falls within the field of view of the sensor over onesegment of each spin. In some cases it may be desirable to have amultiplicity of detectors to obtain a greater composite field of viewwithout reduced accuracy.

Detector 13 is mounted in a housing 14 that is opaque to opticalradiation from the body, except for a pinhole 15 that is provided at oneend of the housing. Positioned behind pinhole 15 is planar,semiconducting detecting surface 16 that has substantial lengths in onedirection for single dimension application, or in two directions atright angles to each other for two dimensional applications. Preferably,the detector surface 16 is included in a commercially available devicefrom a number of sources, such as the SC" series of detectorsmanufactured by United Detector Technology. Surface 16 can either be ofcircular configuration, as illustrated in FIG. 3, or it can be formed asa square. For one dimensional application it is linear. Detector surface16 is provided with a number (two or more) of spaced electrodes forderiving signals indicative of the centroid position of the radiationemitting body, as imaged on the surface by pinhole 115. In one preferredembodiment, detecting surface 16 is a PIN silicon photodiode having avery fast response, ranging from a few nanoseconds up to less thanmicroseconds. Preferably, detecting surface 16 includes four outputelectrodes 21-24, positioned orthogonally to each other on the peripheryof the surface, and a single, centrally located input electrode 25 thatis connected to one terminal of d.c. power supply 26, the terminal ofwhich is grounded.

Detector 16 responds to the relatively small light spot imaged on itssurface by pinhole to derive at terminals 21-24 signals indicative ofthe centroid of the image relative to the electrode positions. Thereby,the difference between the output signals of electrodes 21 and 22provides an indication of the centroid position of the image on detectorsurface 16 in a first coordinate direction (along a Y-axis) and thedifference between the output signals at the electrodes 23 and 24provides an indication of the centroid position of the image in adirection at right angles to the first coordinate direction (along anX-axis).

The magnitude of voltages derived at electrodes 21-24 is also indicativeof the intensity of the radiation impinging on detector surface 16, anddark current flowing in the detector. Dark current is an offset currentvalue that has a d.c. level dependent upon internal conditions, such astemperature and age, of the detector surface 16. The dark current is,thereby, subject to long term fluctuations which cannot be predicted ona predetermined basis. Dark current fluctuations are minimized to acertain extent by stabilizing the temperature of the detector surface16. To this end heater coils 27 are located in proximity to surface 16and supplied with a regulated current.

The area of detector surface 16 and the spacing between the plane inwhich the detector surface is located relative to pinhole 15 are suchthat there is an appreciable field of view (on the order of in a typicalembodiment) for the detector. The field of view, 6, is determined by thediameter of detector surface 16 (L) and the distance of surface 16 frompinhole 15 (d) as: 9 tan L/(Zd). To provide accuracies to within oneminute of arc over the 20 field of view of detector surface 1%, the areaof pinhole 15 is no greater than approximately one-tenth the surfacearea of the detector surface. In one particular embodiment, the pinholediameter is approximately one thirty-second inch and the detectordiameter is approximately three-fourths inch. The maximum and minimumrelationships between the pin and detector areas are governed byaccuracy and signal to noise ratio at each electrode. If the pinholearea is greater than approximately one-tenth of the detector surfacearea the radiation image on the detector surface 16 is excessively largeand may be so diffuse that the ability to drive signals accuratelyrepresenting the centroid position is reduced. If the pinhole area istoo small the total sunlight incident on the photocell will not besufficient to maintain an adequate signal to noise ratio.

In order to provide accurate indications of the relative angularposition between spacecraft 11 and the body, to within 1 minute of arcin the 20 field of view, it is necessary for the output signals derivedfrom electrodes 21-24 to be processed in a manner to accurately removethe dark current offset thereon. To this end, the output signal of eachof electrodes 21-24 is fed to a separate signal processing channel;channels 31-34 being respectively provided for electrodes 21-24. Sinceeach of channels 31-34 is substantially the same, a description ofchannel 31 suffices for the remainder.

While no radiation to which the detecting surface is responsive is inthe field of view of radiation detecting surface 16, electrode 21derives a finite, non-zero d.c. output signal voltage level indicativeof the detector dark current. In response to radiation from the bodyimpinging on the detector surface 16, the voltage at electrode 21increases to a d.c. value equal to the sum of the dark current plus amagnitude that is a function of the intensity of the radiation impingingon the detector surface and the position of the image on the detectorsurface. The two d.c. voltage levels are derived in sequence during eachspin of the spacecraft while detector surface 16 is being illuminated bythe body. Hence, the spin of the spacecraft provides a cyclic choppingof the radiant energy that impinges on detector surface 16.

The output signal of electrode 21 is fed to one input terminal ofdifference node 35 of channel 31. A second input to node 35 isresponsive to a feedback signal derived from the output of channel 31.Node 35 derives a difference signal that is fed to wide band amplifier36 which derives an output signal that is fed through a normally closedswitch 37 to storage capacitor 38. Normally closed switch 37 preferablyis the source drain path of a field effect transistor having a gateelectrode that is normally biased so that a very low impedance existsbetween the source and drain electrodes. Capacitor 38 is connectedthrough isolating amplifier 39 to the second input terminal of node 35,whereby the node derives an output signal that is at all times equal tothe difference between the signal derived from electrode 21 and thesignal stored by capacitor 38. The signal stored by capacitor 38 isindicative of the dark current derived from detector surface 16 sincethe capacitor is connected with the output terminal of amplifier 36 atall times except while detector surface 16 is being illuminated, andthereby remains substantially constant throughout the interval of eachspin of spacecraft 11.

To control switch or field effect transistor 37, the outputs of each ofchannels 31-34, as derived from amplifiers 36 thereof, are linearlycombined in summation network 41. The amplitude of the output signal ofsummation network 41 is thereby directly proportional to the totalradiant energy of the image on detector surface 16. Schmidt trigger 42responds to the output of summation network 41 to derive a signal thatpinches off the source drain path of field effect transistors 37 in eachof channels 31-34 to establish a high impedance, open circuit switchcondition. The Schmidt trigger remains operative as long as the outputsignal of summation amplifier 41 exceeds a predetermined, thresholdvalue indicative of a certain radiation image level on a detectorsurface 16. After radiation from the sun or other celestial body beingtracked is no longer impinging on detector surface 16, the level of theoutput signal of summation circuit 41 drops below the threshold value ofSchmidt trigger 42 and the source drain path of the field effecttransistor 37 in each of channels 31-34 is restored to a low impedance.Thereby, the voltage level on capacitor 38 remains substantiallyconstant throughout the interval of each spin of spacecraft 11 aboutvertical axis 12, at a level commensurate with the dark current derivedfrom electrode 21 to which it is responsive. This voltage level iscontinuously fed through amplifier 39 to difference node 35, whereby theeffect of offset is substantially eliminated in the output of channels31-34 throughout each spin of the spacecraft.

To provide an accurate indication of the angular position of thespacecraft relative to the body, it is preferable to continuouslymonitor the output of each of channels 31-34. Due to modern telemeteringconstraints in spacecraft to earth based communication links, however,it is necessary to sample the output of channels 31-34 and feed thesesamples on a time multiplex basis to a single transmitter. To this end,the output signals of channels 31-34, as derived at the output terminalsof amplifiers 36 in each channel, are fed to sample and hold networks 43of the respective chan nels. The sample and hold networks 43 of each ofchannels 31-34 are simultaneously activated in response to controlsignals from multiplex controller 44 to the sample condition to preventmotion induced errors since all of the signals for a particularmultiplex frame are combined at the ground station to provide anindication of spacecraft position during that frame. The sampling rateof sample and hold networks 43 of channels 31-34 is much greater thanthe spin rate of spacecraft 11 about axis 12 to approximate thecontinuous derivation of signals from electrodes 21-24. In an exemplarycase, spacecraft 11 spins about axis 12 at a rate of six times persecond, while signals are sampled by networks 43 250 times per second inresponse to signals derived by controller 44.

The signals stored or held by networks 43 are sequentially fed atdifferent times during each multiplex frame through time multiplexer 45to a variable gain output channel 50 having a discrete number ofpredetermined gain factors. Time multiplexer 45 is responsive to controlcircuit 46 so that signals are read from sample and hold networks 43 atthe same rate as the networks sample the output signals of channels31-34. Multiplexer 45 feeds the four input signals thereof to a singleoutput lead 47 which drives multiplier 48. Multiplier 48 sets the gainof output channel 50 so that the variable intensity of the radiationemitted by the celestial body, as impinging on detector 16, is partiallyremoved from the output signal of channel 50. In this manner, almost thefull dynamic range of the output signal is maintained for wide range oflight levels.

Multiplier 48 functions as a variable gain device having a signal inputfrom multiplexer 45 and a gain control input responsive to controller49, that has a number of predetermined, selectable output levelsdetermined by the known intensity of the radiation image detected bydetector surface 16. If detector surface 16 is responsive to radiationfrom the sun, multiplier 48 can be omitted since solar radiant energyimpinging on surface 16 can be considered constant. If, however, thedetector surface 16 is responsive to lunar radiant energy images,controller 49 introduces variable gain factors to control the amplitudeof the multiplication factor of multiplier 48. The magnitude of the gainfactors depends on the lunar phase. The controller 49 utilizes the sumof outputs of the sample and hold circuits to determine the requiredgain setting by comparing the product of the sum and the gain setting toa full scale output, and adjusting the gain until proper signal levelsare obtained.

There is a tendency for offset to be introduced by time multiplexer 45and the circuitry of channel 50. To eliminate such offset, outputchannel 50 is provided with an electronic circuit similar to thatemployed in each of channels 31-34. The output channel offset removingcircuit includes differential or subtraction node 52, having one inputresponsive to the output of multiplier 48 and a second input responsiveto a feedback signal derived from isolating amplifier 53. The differenceoutput signal of node 52 drives an input terminal of amplifier 54, theoutput of which is connected through the source drain path of fieldeffect transistor to storage capacitor 56. Field effect transistor 55includes a gate electrode responsive to control network 46 for timemultiplexer 45. When control network 46 activates multiplexer 45 so thatno signals are fed through the multiplexer to lead 47, it biases thegate electrode of field effect transistor 55 so that low impedance isprovided between the source drain electrodes of the field effecttransistor, whereby capacitor 56 is responsive to any finite, non-zerovoltage that might be derived at the output terminal of amplifier 54during this interval. When control noetwork 46 activates multiplexer 45so that the output signal of one of channels 31-34 is fed through themultiplexer to multiplier 48, it applies a bias voltage to the gateelectrode of field effect transistor 55 to pinch off the field effecttransistor source drain path. Thereby, the finite, signal levelindicating output voltage of amplifier 54 is decoupled from capacitor 56so that the capacitor voltage is unaffected by the finite positionindicating signal level derived from multiplier 48 and the capacitorcontinuously feeds a voltage to node 52 indicative of the finite offsetvoltage of multiplexer 54 and channels 50.

To obtain accuracies to within one minute of are within a 20 field ofview, it is been found necessary to calibrate presently availablecommercial semiconductor devices employed as detector surface 16.Calibration is performed by placing detector 13 on a calibration tablewith a suitable light source and incrementally rotating the detector sothat the detector surface 16 is moved to a very large number of pointsin the 20 field of view. In one calibration scheme, 1,600 differentpoints were actually employed. To provide the desired accuracy, theoutput signals of amplifier 54, as sequentially derived from channels31-34 for each for the sampled points in the field of view, are combinedto derive positional information for the X and Y-axes in accordancewith:

(l)and where:

V, equals the output voltage of amplifier S4 for channel 33; V equalsthe output voltage of amplifier 54 for channel 34; V, equals the outputvoltage of amplifier 54 for channel 31; and V equals the output voltageof amplifier 54 for channel 32. In Equations (l) and (2) there isincluded a denominator factor proportional to the sum of the responsesof channels 31-34 while detector surface 16 is illuminated to providenormalization of the different signals for intensity variations of thelight source illuminating surface 116.

Depending upon the accuracy required, calibration may be performed byanalog or digital techniques. Data derived from detector 16 duringactual use while it is desired to measure the position of spacecraft l1relative to the sun or moon can be compared with the calibration data toderive the position information by manual or computer techniques.

While there has been described and illustrated one specific embodimentof the invention, it will be clear that variations in the details of theembodiment specifically illustrated and described may be made withoutdeparting from the true spirit and scope of the invention as defined inthe appended claims.

We claim:

1. A device for deriving signals indicative of the relative angularposition of a radiation emitting celestial body and a spinningspacecraft comprising a detector for the radiation, means for forming aradiation image of the body on the detector, said image having an areamuch less than the detecting surface of the detector, said detectorderiving a first signal having a first nonzero level while said detectoris not illuminated by said radiation and a second non-zero level whileit is illuminated by said radiation, said first level being subject tochange in response to internal conditions of the detector, said secondlevel being dependent upon: the internal conditions, the position of thecentroid of said image on the detector surface, and the intensity ofsaid radiation impinging on the detector; said first and second levelsbeing derived in sequence during each spin of the spacecraft, meansresponsive during each spin to the first signal for deriving a secondsignal having a level proportional to the first level, said secondsignal remaining substantially constant throughout the interval of eachspin, and means combining the first and second signals for deriving athird signal having a substantially zero level while the first level isbeing derived and a third level substantially proportional to thedifference between the first and second levels while the second level isbeing derived.

2. The device of claim 1 further including means for multiplying thethird signal by a level indicative of the intensity of the source.

3. The device of claim 2 wherein the source is subject to intensityvariations, and means for changing the multiplication level as afunction of the intensity variations.

4. The device of claim 1 wherein said image forming means is a pinholehaving an area no greater than approximately one-tenth the area of thedetecting surface.

5. A device for deriving signals indicative of the relative angularposition of a radiation emitting celestial body and a spinningspacecraft comprising a detector for the radiation, means for forming aradiation image of the body on the detector, said image having an areamuch less than the detecting surface of the detector, said detectorhaving a planar detecting surface with substantial lengths in twodirections at right angles to each other, said detector including aplurality of output electrodes, said plurality being greater than two,said electrodes being spaced relative to said surface so that at each ofthem there is derived a signal having a first non-zero level while saiddetector is not illuminated by said radiation and a second non-zerolevel while it is illuminated by said radiation, said first level beingsubject to change in response to internal conditions of the detector,said second level being dependent upon: the internal conditions, theposition of the centroid of said image on the detector surface relativeto the electrode, and the intensity of said radiation impinging on thedetector; the relative amplitudes of the second levels derived from theplural electrodes providing an indication of the position of thedetector and image in two directions at right angles to each other, saidfirst and second levels being derived in sequence during each spin ofthe spacecraft, means, for each electrode, responsive during each spinto the first signal for deriving a second signal having a levelproportional to the first level, said second signal remainingsubstantially constant throughout the interval of each spin, and means,for each electrode, combining the first and second signals for derivinga third signal having a substantially zero level while the first levelis being derived and a third level substantially proportional to thedifference between the first and second levels while the second level isbeing derived.

6. The device of claim 5 wherein the second signal deriving means foreach electrode includes: signal storage means normally responsive tosaid first signal, and means responsive to the detector beingilluminated by the source for decoupling the first signal from thestorage means.

7. The device of claim 5 wherein each of the signals is d.c., and thesecond and third signal deriving means for each electrode includes: afeedback network having an input responsive to the first signal, meansresponsive to the first and second signals for deriving a dc. differencesignal, means for amplifying the difference signal to derive the thirdsignal, a storage capacitor for deriving the second signal, switch meansfor normally feed ing the third signal to the storage capacitor, andmeans responsive to the detector being illuminated by the source foractivating the switch means to prevent the third signal from being fedto the storage capacitor while the detector is being illuminated by thebody.

8. The device of claim 7 wherein the means for activating is responsiveto the third signals for all of said electrodes.

9. The device of claim 5 further including means for periodicallysampling, at a frequency much greater than the spin frequency, the thirdsignal for all of said electrodes at substantially the same time.

10. The device of claim 9 further including means for separately storingthe sampled signal for each electrode, means for time multiplexing thestored sampled signals, and a single variable gain output channelresponsive to the time multiplexing means, said output channel includingmeans for multiplying each of the third signals by a level indicative ofthe intensity of the source.

11. The device of claim 10 wherein the source is subject to intensityvariations, and means for changing the multiplication level as afunction of the intensity variations.

12. The device of claim 11 wherein said output channel introduces anoffset so that it derives a signal having a finite, non-zero value whilenone of the third signals is fed through the multiplexing means, andmeans responsive to the signal derived by the channel for removing theoffset from it.

13. The device of claim 5 wherein said image forming means is a pinholehaving an area no greater than approximately one-tenth the area of thedetecting surface.

14. A device for deriving signals indicative of the relative angularposition of a radiation emitting celestial body and a spacecraftcomprising a detector for the radiation, means for forming a radiationimage of the body on the detector, said image having an area much lessthan the detecting surface of the detector, said detector deriving afirst signal having a first non-zero level while said detector is notilluminated by said radiation and a second non-zero level while it isilluminated by said radiation, said first level being subject to changein response to internal conditions of the detector, said second levelbeing dependent upon: the internal conditions, the position of thecentroid of said image on the detector surface, and the intensity ofsaid radiation impinging on the detector; said spacecraft includingmeans for cyclically chopping the radiation illuminating the detector sothat said first and second values are derived in sequence during eachchopping cycle while the radiation is respectively blocked and passed bythe chopping'means, means responsive only to the first signal forderiving a second signal having a level proportional to the first level,said second signal remaining substantially constant throughout acomplete chopping cycle, and means combining the first and secondsignals for deriving a third signal having a substantially zero levelwhile the first level is being derived and a third level substantiallyproportional to the difference between the first and second levels whilethe second level is being derived.

15. A system for deriving signals indicative of the angular position ofa radiation emitting celestial body and a spacecraft comprising adetector for the radiation, means for forming a radiation image of thebody on the detector, said image having an area much less than thedetecting surface of the detector, said detector having a planardetecting surface with substantial lengths in two directions at rightangles to each other, said detector including a plurality of outputelectrodes, said plurality being greater than two, said electrodes beingspaced relative to said surface so that at each of them there is deriveda signal having a first non-zero level while said detector is notilluminated by said radiation and a second non-zero level while it isilluminated by said radiation, said first level being subject to changein response to internal conditions of the detector, said second levelbeing dependent upon: the internal conditions, the position of thecentroid of said image on the detector surface relative to theelectrode, and the intensity of said radiation impinging on thedetector; the relative amplitudes of the second levels derived from theplural electrodes providing an indication of the position of thedetector and image in two directions at right angles to each other, saidspacecraft including means for cyclically chopping the radiationilluminating the detector so that said first and second values arederived in sequence during each chopping cycle while the radiation isrespectively blocked and passed by the chopping means, means responsiveonly to the first signal for deriving a second signal having a levelproportional to the first level, said second signal remainingsubstantially constant throughout a complete chopping cycle, and means,for each electrode, combining the first and second signals for derivinga third signal having a substantially zero level while the first levelis being derived and a third level substantially proportional to thedifference between the first and second levels while the second level isbeing derived.

16. A device for deriving signals indicative of the angular position ofa radiation emitting celestial body comprising a spacecraft, a detectorfor the radiation positioned on the spacecraft, means for forming aradiation image of the body on the detector, said image having an areamuch less than the detecting surface of the detector, said detectorhaving a planar detecting surface with substantial lengths in twodirections at right angles to each other, said detector including aplurality of output electrodes being spaced relative to said surface sothat at each of them there is derived a signal having a first non-zerolevel while said detector is not illuminated by said radiation and asecond non-zero level while it is illuminated by said radiation, saidfirst level being subject to change in response to internal conditionsof the detector, said second level being dependent upon: the intemalconditions, the position of the centroid of said image on the detectorsurface relative to the electrode, and the intensity of said radiationimpinging on the detector, the relative amplitudes of the second levelsderived from the plural electrodes providing an indication of theposition of the detector and image in two directions at right angles toeach other, wherein said image forming means is a pinhole having an areano greater than approximately onetenth the area of the detectingsurface.

1. A device for deriving signals indicative of the relative angular position of a radiation emitting celestial body and a spinning spacecraft comprising a detector for the radiation, means for forming a radiation image of the body on the detector, said image having an area much less than the detecting surface of the detector, said detector deriving a first signal having a first non-zero level while said detector is not illuminated by said radiation and a second non-zero level while it is illuminated by said radiation, said first level being subject to change in response to internal conditions of the detector, said second level being dependent upon: the internal conditions, the position of the centroid of said image on the detector surface, and the intensity of said radiation impinging on the detector; said first and second levels being derived in sequence during each spin of the spacecraft, means responsive during each spin to the first signal for deriving a second signal having a level proportional to the first level, said second signal remaining substantially constant throughout the interval of each spin, and means combining the first and second signals for deriving a third signal having a substantially zero level while the first level is being derived and a third level substantially proportional to the difference between the first and second levels while the second level is being derived.
 2. The device of claim 1 further including means for multiplying the third signal by a level indicative of the intensity of the source.
 3. The device of claim 2 wherein the source is subject to intEnsity variations, and means for changing the multiplication level as a function of the intensity variations.
 4. The device of claim 1 wherein said image forming means is a pinhole having an area no greater than approximately one-tenth the area of the detecting surface.
 5. A device for deriving signals indicative of the relative angular position of a radiation emitting celestial body and a spinning spacecraft comprising a detector for the radiation, means for forming a radiation image of the body on the detector, said image having an area much less than the detecting surface of the detector, said detector having a planar detecting surface with substantial lengths in two directions at right angles to each other, said detector including a plurality of output electrodes, said plurality being greater than two, said electrodes being spaced relative to said surface so that at each of them there is derived a signal having a first non-zero level while said detector is not illuminated by said radiation and a second non-zero level while it is illuminated by said radiation, said first level being subject to change in response to internal conditions of the detector, said second level being dependent upon: the internal conditions, the position of the centroid of said image on the detector surface relative to the electrode, and the intensity of said radiation impinging on the detector; the relative amplitudes of the second levels derived from the plural electrodes providing an indication of the position of the detector and image in two directions at right angles to each other, said first and second levels being derived in sequence during each spin of the spacecraft, means, for each electrode, responsive during each spin to the first signal for deriving a second signal having a level proportional to the first level, said second signal remaining substantially constant throughout the interval of each spin, and means, for each electrode, combining the first and second signals for deriving a third signal having a substantially zero level while the first level is being derived and a third level substantially proportional to the difference between the first and second levels while the second level is being derived.
 6. The device of claim 5 wherein the second signal deriving means for each electrode includes: signal storage means normally responsive to said first signal, and means responsive to the detector being illuminated by the source for decoupling the first signal from the storage means.
 7. The device of claim 5 wherein each of the signals is d.c., and the second and third signal deriving means for each electrode includes: a feedback network having an input responsive to the first signal, means responsive to the first and second signals for deriving a d.c. difference signal, means for amplifying the difference signal to derive the third signal, a storage capacitor for deriving the second signal, switch means for normally feeding the third signal to the storage capacitor, and means responsive to the detector being illuminated by the source for activating the switch means to prevent the third signal from being fed to the storage capacitor while the detector is being illuminated by the body.
 8. The device of claim 7 wherein the means for activating is responsive to the third signals for all of said electrodes.
 9. The device of claim 5 further including means for periodically sampling, at a frequency much greater than the spin frequency, the third signal for all of said electrodes at substantially the same time.
 10. The device of claim 9 further including means for separately storing the sampled signal for each electrode, means for time multiplexing the stored sampled signals, and a single variable gain output channel responsive to the time multiplexing means, said output channel including means for multiplying each of the third signals by a level indicative of the intensity of the source.
 11. The device of claim 10 wherein the source is subject to intensity variations, And means for changing the multiplication level as a function of the intensity variations.
 12. The device of claim 11 wherein said output channel introduces an offset so that it derives a signal having a finite, non-zero value while none of the third signals is fed through the multiplexing means, and means responsive to the signal derived by the channel for removing the offset from it.
 13. The device of claim 5 wherein said image forming means is a pinhole having an area no greater than approximately one-tenth the area of the detecting surface.
 14. A device for deriving signals indicative of the relative angular position of a radiation emitting celestial body and a spacecraft comprising a detector for the radiation, means for forming a radiation image of the body on the detector, said image having an area much less than the detecting surface of the detector, said detector deriving a first signal having a first non-zero level while said detector is not illuminated by said radiation and a second non-zero level while it is illuminated by said radiation, said first level being subject to change in response to internal conditions of the detector, said second level being dependent upon: the internal conditions, the position of the centroid of said image on the detector surface, and the intensity of said radiation impinging on the detector; said spacecraft including means for cyclically chopping the radiation illuminating the detector so that said first and second values are derived in sequence during each chopping cycle while the radiation is respectively blocked and passed by the chopping means, means responsive only to the first signal for deriving a second signal having a level proportional to the first level, said second signal remaining substantially constant throughout a complete chopping cycle, and means combining the first and second signals for deriving a third signal having a substantially zero level while the first level is being derived and a third level substantially proportional to the difference between the first and second levels while the second level is being derived.
 15. A system for deriving signals indicative of the angular position of a radiation emitting celestial body and a spacecraft comprising a detector for the radiation, means for forming a radiation image of the body on the detector, said image having an area much less than the detecting surface of the detector, said detector having a planar detecting surface with substantial lengths in two directions at right angles to each other, said detector including a plurality of output electrodes, said plurality being greater than two, said electrodes being spaced relative to said surface so that at each of them there is derived a signal having a first non-zero level while said detector is not illuminated by said radiation and a second non-zero level while it is illuminated by said radiation, said first level being subject to change in response to internal conditions of the detector, said second level being dependent upon: the internal conditions, the position of the centroid of said image on the detector surface relative to the electrode, and the intensity of said radiation impinging on the detector; the relative amplitudes of the second levels derived from the plural electrodes providing an indication of the position of the detector and image in two directions at right angles to each other, said spacecraft including means for cyclically chopping the radiation illuminating the detector so that said first and second values are derived in sequence during each chopping cycle while the radiation is respectively blocked and passed by the chopping means, means responsive only to the first signal for deriving a second signal having a level proportional to the first level, said second signal remaining substantially constant throughout a complete chopping cycle, and means, for each electrode, combining the first and second signals for deriving a third signal having a substantially zero level while the firsT level is being derived and a third level substantially proportional to the difference between the first and second levels while the second level is being derived.
 16. A device for deriving signals indicative of the angular position of a radiation emitting celestial body comprising a spacecraft, a detector for the radiation positioned on the spacecraft, means for forming a radiation image of the body on the detector, said image having an area much less than the detecting surface of the detector, said detector having a planar detecting surface with substantial lengths in two directions at right angles to each other, said detector including a plurality of output electrodes being spaced relative to said surface so that at each of them there is derived a signal having a first non-zero level while said detector is not illuminated by said radiation and a second non-zero level while it is illuminated by said radiation, said first level being subject to change in response to internal conditions of the detector, said second level being dependent upon: the internal conditions, the position of the centroid of said image on the detector surface relative to the electrode, and the intensity of said radiation impinging on the detector, the relative amplitudes of the second levels derived from the plural electrodes providing an indication of the position of the detector and image in two directions at right angles to each other, wherein said image forming means is a pin hole having an area no greater than approximately one-tenth the area of the detecting surface. 